A review of open-field host range testing to evaluate non-target use by herbivorous biological control candidates

One of the fundamental challenges of pre-release studies in classical biological weed control is to assess and predict the likelihood and consequences of non-target effects. Unless a candidate biological control agent is proven to be monophagous through conventional starvation and host-specificity tests in quarantine, open-field host range studies can be important in predicting the likelihood of non-target effects since they reveal the host selection of herbivores displaying the whole array of pre- and post-alightment behaviours. Over the course of its 53-year history, the purpose and the design of open-field host range studies have changed considerably, with more recent studies clarifying or refining specific questions related to one or a few test plant species and using a set design. We discuss the opportunities and challenges of this approach and suggest that future open-field host range studies should be more hypothesis-driven and apply different experimental designs that facilitate the interpretation of the results.